The following generally relates to liquid crystal (LC) devices and methods for applications such as, but is not limited to, LC electro-optical devices, tunable diffraction gratings, color filters, light deflectors and scatterers, wide-angle beam steerers, and the like.
A state matter called nematic liquid crystal (LC) is defined by an orientationally ordered fluid having an average orientation of nematic molecules described by a so-called director ({circumflex over (n)}). The best known and most widely used nematic LC material in modern LC display applications is the uniaxial nematic LC. In uniaxial nematic LCs, rod-like achiral molecules are aligned along a single straight axis which serves as the director. If some or all of these nematic molecules are chiral instead of achiral, this director will twist in space and thus follow a right-angle helix. This twisting of the director results in a structure of nematic molecules referred to as either chiral nematic (N*) or cholesteric LC state.
Chiral nematics are highly promising for active photonic applications, e.g., for use in displays, tunable lasers, energy-conserving windows, and tunable color filters. This is due, in part, to the selective reflection of N* and their self-organized right-angle helical director field. The selective reflection is a manifestation of the periodic helical organization of the cholesteric phase. When macroscopically organized in the Grandjean texture (uniform standing helix), the chiral nematic satisfies the condition for a reflection of light as defined by the Bragg Equation. For light propagating parallel to the helical axis, the central wavelength of the reflection bandgap is defined as: λp=nP, where P is the pitch length of the helical twist of the director and n is the average refractive index of the liquid crystal. Assuming a constant pitch, the reflection bandwidth of N* is defined by Δλ=ΔnP, where Δn is the birefringence of the LC.
The reflection color and reflectivity of N* can be controlled by a variety of stimuli including electric field, heat, and light. The field-induced modification of the helix of the N* material is typically performed by one of two ways: (1) changing the pitch of the helix, e.g., such as in diffractive element applications, or (2) realigning the helix axis as the whole, e.g., as used in bistable displays. In both of these methods, the fundamental character of the helical twist remains intact.
However, direct application of an electric field to cholesteric LCs to control the reflected color presents many problems. Once the electric field is applied, the periodic structure of N* becomes distorted in a non-uniform manner and induces an non-uniform coloration and reflectivity. In the case of an electric field applied parallel to the helix axis, the main reason for the disruption of the N* periodic structure is out-of-plane rotation of the helix from the uniform planar state to the disordered focal conic state. Unwinding of the helix using an electric field perpendicular to the helix axis can lead to a change in the wavelength of the reflection band, but such an approach requires fringe-fields which locally distort the homogeneity of the structure. Helfrich deformation, comprising undulations of common director orientation planes parallel to the substrates, has also been shown to provide a means of change the reflection color within a narrow tuning range, in which the local tilting of helix leads to shortening of pitch under the normal incidence. However, the undulations lead to spatial inhomogeneity of the pitch causing a broadening of the reflection band and decrease in the reflectance level.